Drawdown apparatus and installation method for a floating platform

ABSTRACT

A floating platform includes a hull and a deck mounted thereon. The platform is anchored to the seabed by tendons connected to the hull at the upper ends thereof and secured to the seabed at the lower ends thereof. The platform includes a removable drawdown system for lowering the platform to the lock-off draft without utilizing an installation vessel.

BACKGROUND OF THE DISCLOSURE

The present invention relates to floating platform systems for testingand producing hydrocarbon formations found in offshore waters. Moreparticularly, the invention relates to a platform drawdown method andsystem for maintaining stability of a floating structure while loweringthe platform down to its installation or lock-off draft.

The exploration for oil and gas deposits in offshore waters, andrecovery of the oil and gas therefrom is very expensive. Various methodsand offshore production systems have been utilized to locate and recoveroffshore oil and gas deposits. Exploration and production systems suchas converted Mobile Offshore Drilling Units (“MODU”), Tension LegPlatforms (TLP) and other floating structures typically used in offshorewaters are very expensive to manufacture and difficult to install.

Installation of an offshore platform, such as a TLP, may require thatthe platform hull be wet towed to the installation site. The hull of asingle column TLP comprises the central column and a plurality ofpontoons extending radially outwardly from the central column. The hullof a single column TLP is quite stable floating on the pontoons as it iswet towed to the installation site because of the large water plane areaprovided by the central column and pontoons. During a typical TLPinstallation, the hull is ballasted down for connection to a pluralityof pre-installed tendons which anchor the hull to the seabed. As thehull is ballasted down, the hydrostatic stability of the platformdecreases significantly as the pontoons submerge below the water surfaceand the platform water plane area at the water surface is substantiallyreduced. To minimize stability problems during installation, the hull istypically installed without the deck and a stabilizing upward force isapplied at the top of the hull as it is ballasted down to connect withthe pre-installed tendons. The upward stabilizing force or hook load istypically supplied by an installation support vessel. After the hull islowered to the lock-off draft, connected to the tendons and deballasted,the platform is sufficiently stable so that the deck may be installed.The installation support vessel or other lifting equipment may be usedto lift the deck and safely set it on the platform hull. However, only alimited number of installation support vessels are available worldwidecapable of providing the hook load required by a typical TLP and therental rates for installation support vessels is very high.

For some offshore platform installations, it may be very advantageouscommercially to utilize an installation alternative that eliminates theneed for expensive installation support vessels during hull and deckinstallation. One such installation method, would for example, includeinstalling the deck on the hull onshore or in a less exposed, shallowerwater location with less expensive lifting means. The hull-deck assemblywould then be wet-towed to the installation site and lowered to itsinstallation draft and connected it to the pre-installed tendons withoutusing an installation support vessel.

It is therefore an object of the present invention to provide a methodfor installing a floating platform without using an installation supportvessel to provide stability to the hull or hull-deck assembly of theplatform during transition of the platform from the wet tow draft to itsinstallation or tendon lock-off draft.

It is a further object of the present invention to provide a method forinstalling a floating platform including the steps of assembling theplatform, including the hull, deck, drawdown system and most productionequipment at or near the fabrication site; wet towing the assembledplatform to the installation site; and actuating the drawdown system forlowering the assembled platform to its installation draft without aninstallation vessel.

It is a further object of the present invention to provide a floatingplatform utilizing a drawdown system to rapidly lower the platform toits installation draft.

SUMMARY OF THE INVENTION

In accordance with a preferred embodiment of the present invention, afloating platform includes a hull and a deck mounted on the uppermostend of the hull. Pontoons extend radially outwardly from the lower endof the hull. The platform is anchored to the seabed by a plurality oftendons connected to the hull at the upper ends thereof and secured tothe seabed at the lower ends thereof. The platform includes a removabledrawdown system for lowering the platform to lock-off draft withoututilizing an installation vessel.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features, advantages andobjects of the present invention are attained can be understood indetail, a more particular description of the invention brieflysummarized above, may be had by reference to the embodiments thereofwhich are illustrated in the appended drawings.

It is noted, however, that the appended drawings illustrate only typicalembodiments of this invention and are therefore not to be consideredlimiting of its scope, for the invention may admit to other equallyeffective embodiments.

FIG. 1 is a side view of a floating platform illustrating the platformat the wet tow draft positioned above pre-installed tendons andrigged-up in accordance with the invention;

FIG. 2 is a side view of the floating platform illustrating a step inthe installation method of the invention wherein the drawdown system isattached to the tendons and the platform is transitioning between thewet tow draft and the installation draft;

FIG. 3 is a side view of the floating platform illustrating a step inthe installation method of the invention wherein the tendons have passedthrough the tendon connectors and the platform is ready for lock-off;

FIG. 4 is a side view of the floating platform at lock-off draft and thedrawdown system has been dismantled and recovered;

FIG. 5 is a side view of the drawdown system of the invention;

FIG. 6 is an end view of the drawdown system of the invention;

FIG. 7 illustrates the tensioning adapter member of the drawdown systemof the invention;

FIG. 8 is a side view of a floating platform illustrating the platformat the wet tow draft positioned above the pre-installed tendons andrigged-up in accordance with an alternate embodiment of the drawdownsystem of the invention;

FIG. 9 is an enlarged partial side view of the drawdown system of theinvention illustrated in FIG. 8;

FIG. 10 is an enlarged partial side view of the floating platformillustrating a step in the installation method of the invention whereinthe drawdown system illustrated in FIG. 8 is attached to the tendons andthe platform is ready to be lowered to the lock-off draft;

FIG. 11 is a side view of the floating platform illustrated in FIG. 8 atlock-off draft;

FIG. 12 is a side view of the floating platform illustrating a step inthe installation method of an alternate embodiment of the inventionwherein the drawdown system is mounted on temporary stability columnssecured to the platform and the platform is transitioning to theinstallation draft; and

FIG. 13 is a side view of the floating platform illustrating a step inthe installation method of the invention illustrated in FIG. 12 whereinthe tendons have passed through the tendon connectors and the platformis ready for lock-off.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring first to FIG. 1, the floating platform of the invention isgenerally identified by the reference numeral 10. The platform 10includes a hull 12 which provides positive buoyancy and vertical supportfor the platform deck 14. In a preferred embodiment of the inventionillustrated in FIG. 1, the hull 12 comprises a central column 13 andpontoons 18 extending radially outwardly from the lower end of thecentral column 13. One or more decks 14 are supported on the centralcolumn 13. Drilling and/or production equipment necessary for therecovery and processing of oil, gas and water are secured on the decks14.

The central column 13 extends upward from the base or keel of theplatform 10. The base node of the platform 10 is at the intersection ofthe central column 13 and the pontoons 18 extending radially outwardlytherefrom. The platform 10 is anchored to the seabed by tendons 20secured at one end thereof to the pontoons 18, as shown in FIG. 3, andat the opposite ends thereof to foundation piles (not shown in thedrawings) embedded in the seabed.

In accordance with the present invention, the platform 10 includes adrawdown system mounted on each pontoon 18. The drawdown system includesstrand jacks, generally identified by the reference numeral 28,temporarily mounted on the distal ends of the pontoons 18. The strandjacks 28 include tower masts 30 mounted on the pontoons 18 and extendingupwardly therefrom. The tower masts 30 comprise one or more tower mastsections 32 vertically stacked one on top of the other to the designheight. The height of the tower masts 30 is sufficient to maintain thetops thereof above the water surface at the installation draft of theplatform 10 and provide some hydrostatic stability for the platform 10at all intermediate drafts as it is lowered to the lock-off draft. Thelowermost mast section 32 is mounted on a mast base 34 secured to thetop surface of the pontoons 18. Oppositely facing towers 30 are erectedon each of the pontoons 18 and are interconnected by cross-ties 36 toform a rigid drawdown support structure. Additional support is providedby brace members 38 extending at an angle from the tower masts 30 to thepontoons 18 and secured thereto. A strand jack base 40 is mounted acrossthe tops of the oppositely facing tower masts 30. Strand jack controls42 for each strand jack 28 are mounted on the base 40.

In the preferred embodiment shown in FIG. 1, the drawdown system of theinvention utilizes six strand jacks 28, one for each tendon 20. Strandjacks are commercially available lifting jacks used for heavy liftingoperations. Each strand jack 28 includes a bundle of strand cables 44that passes through a strand guide or umbrella 46 and is connected to alifting block 48, shown in greater detail in FIG. 7. The strand jacks 28are individually controlled by the controllers 42 which are linked forequalizing the load among them.

A tensioning adapter 50 is connected to the lower end of the liftingblock 48. The tensioning adapter 50 is sized to pass through the tendonconnection porch 21. An internal connector 52 (floating ball type,J-lock, internal shear keys or similar type connector) is fixed to thelower end of the tensioning adapter 50. The connector 52 is adapted formating engagement with a length adjustment joint 54 which is welded orotherwise secured to the upper ends of the tendons 20. The lengthadjustment joint 54 is externally threaded or grooved and adapted toextend through the tendon porch 21. A tendon lock off connector assemblymounted on the length adjustment joint 54 permits adjustment of thetension of the tendons 20.

Referring now to FIGS. 1-5, the installation sequence will be described.The platform 10, including the hull 12, the deck 14 mounted on top ofthe central column 13 and the drawdown system mounted on the pontoons18, is wet-towed to the installation site and maneuvered over thetendons 20 which have been pre-installed and connected at the lower endsthereof to foundation piles embedded in the seabed. In the configurationshown in FIG. 1, the hull 12 and deck 14 mounted thereon are stablefloating on the pontoons 18 because of the large water plane areaprovided by the pontoons 18. The assembled hull 12 and deck 14, however,tends to be unstable at the transition draft where the pontoons 18 arefully submerged below the water line 16. While the tower masts 30 andthe strand cables 44 provide some buoyancy and stability for theplatform 10 by providing additional water plane areas, the assembledhull 12 and deck 14 is not fully stable and out of danger of capsizinguntil the tendons 20 are connected to the pontoons 18. In the finalinstalled configuration of the platform 10, the tendons 20 providestability for the platform 10.

Once the platform 10 is positioned over the tendons 20, the tensioningadapters 50 are lowered through the tendon porches 21 and connected tothe length adjustment joints 54 of the tendons 20. After all tensioningadapters 50 have been connected to the tendons 20, the platform 10 maybe lowered to the installation draft. In operation, each of the strandcables 44 of the drawdown system is fed through top and bottom anchorheads or slips (not shown in the drawings) that open and close torelease or grip the strand cables 44. Initially, the bottom anchors ofthe strand jacks 28 are opened and release the strand cables 44. Anupward lifting force is simultaneously applied to the top anchors of thestrand jacks 28 drawing the strand cables 44 with them. At the top ofthe stroke, the bottom anchors of the strand jacks 28 are closed andgrip the strand cables 44. The top anchors are then opened and loweredto their starting position. The cycle is repeated until the platform 10is lowered to the installation draft and the length adjustment joints 54extend through the tendon porches 21. The tendons 20 are then locked offand the tension in the strand cables 44 is transferred to the tendons 20and tendon porches 21. The platform 10 may be de-ballasted to increasethe tension in the tendons 20 to the installation tension, if necessary,to complete the lock off operation. The tensioning adapters 50 areuncoupled from the length adjustment joints 54 and the drawdown systemis dismantled and removed.

In another embodiment of the invention illustrated in FIGS. 8-11, thedrawdown system of the invention is similar to that described in FIG. 1with the exception that the drawdown system for lowering the platform 10to its lock-off draft includes ratchet connectors and hydraulic cylindertensioning assemblies, one for each tendon 20. The tensioningassemblies, as best shown in FIGS. 9 and 10, include vertically orientedhydraulic cylinders 60 secured at the lower ends thereof to the tendonconnection porches 21. The opposite ends of the hydraulic cylinders 60are connected to dual ratchet connectors 62. Two or more hydrauliccylinders 60 support each of the dual ratchet connectors 62.

A lateral guide structure comprising two or more upright posts 64 and anangular brace member 66 support guide sleeves 68 above the dual ratchetconnectors 62. The guide sleeves 68 are spaced from and verticallyaligned with the dual ratchet connectors 62 so that upon assembly thetensioning adapters 50 extend through the guide sleeves 68 and the dualratchet connectors 62 as illustrated in FIGS. 9 and 10. The tensioningassemblies are individually controlled by the controllers (not shown inthe drawings) which are linked for equalizing the load among them.

The installation sequence utilizing the hydraulic cylinder drawdownsystem is similar to that described above and depicted in FIGS. 1-4.Once the platform 10 is positioned over the tendons 20, the tensioningadapters 50 are lowered through the tendon porches 21 and connected tothe length adjustment joints 54 of the tendons 20. After all tensioningadapters 50 have been connected to the tendons 20, the platform 10 maybe lowered to the installation draft. The hydraulic stroking cylinders60 are activated to simultaneously apply an upward force against thedual ratchet connectors 62. The upward lifting force causes the dualratchet connectors 62 to grip the tensioning adapters 50 and therebyapply an opposite downward force for lowering the platform 10. At thetop of the cylinder stroke, the direction of the stroking cylinders 60is reversed and the dual ratchet connectors 62 disengage from thetension adapters 50. Simultaneously therewith, slip bowl connectorassemblies supported by the tendon porches 21 grip and hold thetensioning adapters 50 while the cylinders 60 return to their startingposition. The cycle is repeated until the platform 10 is lowered to theinstallation draft. The tendons 20 are then locked off and the platform10 may be de-ballasted to increase the tension in the tendons 20 to theinstallation tension, if necessary, to complete the lock off operation.

In another embodiment of the invention illustrated in FIGS. 12 and 13,the drawdown system of the invention is similar to that described inFIG. 1 with the exception that the drawdown system for lowering theplatform 10 to its lock-off draft is mounted on temporary stabilitycolumns 70 supported on the pontoons 18 of the platform 10 at the distalends thereof. The stability columns 70 are floodable and provided withthe necessary plumbing, including a fill port and vent, for connectionwith a ballast system used during installation of the platform 10. Pinand bracket connectors 72 secure the stability columns 70 to thepontoons 18. A strand jack platform 74 secured on top of each of thestability columns 70 provides a mounting platform for the drawdownsystem of the invention. The width of the platforms 74 is greater thanthe width of the stability columns 70 so that they extend sufficientlybeyond the sidewalls of the stability column 70 to enable alignment ofthe strand cables 44 with the tendon porches 21. The strand jackcontrols 42 for each strand jack 28 are secured on the platforms 74.

The alternate embodiment of the invention illustrated in FIGS. 12 and 13further includes one or more heave compensation devices 76. The heavecompensation devices 76 may comprise a plurality of hydraulic cylindersor the like mounted between the strand jack platforms 74 and each of thestrand jack bases 40. The heave compensation devices 76 aid to eliminateoverloading of the drawdown system of the invention in sea statesexceeding the installation criteria for the platform 10. Additionally,the heave compensation devices 76 facilitate transitioning of theplatform 10 from a floating structure to a tension leg platform in acontrolled manner.

The installation sequence utilizing the temporary stability columns 70is substantially the same to that utilizing the tower masts 30 describedabove. In the event additional height is required to maintain the strandjacks 28 above the water line, mast sections 32 may be mounted on thetemporary stability columns 70, as required, to form strand jack towers30 of sufficient height extending above the temporary stability columns70. Upon completion of the alternate embodiment installation procedureillustrated in FIGS. 12 and 13 and lock-off of the platform 10 at theinstallation draft, the drawdown system of the invention and thetemporary stability columns 70 are dismantled and removed.

While a preferred embodiment of the invention has been shown anddescribed, other and further embodiments of the invention may be devisedwithout departing from the basic scope thereof, and the scope thereof isdetermined by the claims which follow.

1. A method for mooring a floating platform to a plurality of tendons,the method comprising the steps of: a) anchoring lower ends of saidtendons to the seabed; b) coupling drawdown tension members mounted onsaid platform to upper ends of said tendons; and c) tensioning saiddrawdown tension members to lower said platform to said tendons.
 2. Themethod of claim 1 including the step of drawing down said platformwithout ballasting said platform.
 3. The method of claim 1 including thestep of concurrently ballasting said platform.
 4. The method of claim 1including the step of passing said drawdown tension members throughtendon porches and connecting said drawdown tension members to saidupper ends of said tendons.
 5. The method of claim 4 including the stepof continuing to tension said drawdown tension members to submerge saidplatform to a lock-off draft and locking off said tendons.
 6. The methodof claim 5 including the step of removing said drawdown tension membersfrom said platform.
 7. The method of claim 6 including the step ofdeballasting said platform to increase the tension in said tendons to aninstallation tension.
 8. The method of claim 1 wherein said platformincludes guide sleeves vertically aligned above tendon porches locatedon said platform and further including the step of passing said drawdowntension members through said guide sleeves and said tendon porches andconnecting said drawdown tension members to said upper ends of saidtendons.
 9. The method of claim 8 including the step of deballastingsaid platform to increase the tension in said tendons to an installationtension.
 10. A floating platform, comprising: a) a hull having an upperend and a lower end; b) a deck supported on said upper end of said hull;c) pontoons secured to said lower end of said hull; d) drawdown tensionmembers mounted on said pontoons; and e) tendons anchoring said floatingplatform to the seabed.
 11. The platform of claim 10 wherein saiddrawdown tension members are adapted for removal from said platform uponlowering said platform to the installation draft.
 12. The platform ofclaim 10 wherein said drawdown tension members comprise strand jacksadapted for connection to upper ends of said tendons.
 13. The platformof claim 10 wherein said drawdown tension members comprise hydrauliccylinder assemblies adapted for connection to upper ends of saidtendons.
 14. The platform of claim 13 wherein said drawdown tensionmembers includes guide sleeves vertically aligned above tendon porcheslocated on said pontoons.
 15. The platform of claim 10 includingtemporary stability columns supporting said drawdown tension members onsaid pontoons.
 16. The platform of claim 15 including heave compensationdevices operatively connected to said drawdown tension members.
 17. Theplatform of claim 10 including heave compensation devices operativelyconnected to said drawdown tension members.
 18. A method for mooring afloating platform to a plurality of tendons, the method comprising thesteps of: a) anchoring lower ends of said tendons to the seabed; b)coupling drawdown tension members mounted on said platform to upper endsof said tendons; c) tensioning said drawdown tension members to submergesaid platform to a lock-off draft; d) locking off said platform at thelock-off draft; and e) dismantling and removing said drawdown tensionmembers from said platform.
 19. The method of claim 18 including thestep of heave compensation for preventing overloading of said drawdowntension members.